The non rigid registration technique, in which a converting function that causes spatial positions of subjects within two three dimensional images that are obtained by imaging a single subject using the same or different imaging apparatuses (modalities) at different timings to match is estimated, and the two types of images are registered by deforming one of the images using the estimated converting function, is being focused on in the field of image diagnosis. The non rigid registration technique sets control points that section image spaces at predetermined intervals. The positions of the control points are displaced, and an amount of image deformation at which the value of an evaluation function that evaluates the degree of similarity among pixel values of a deformed first image and a second image becomes maximal is determined. The converting function is estimated based on the amount of image deformation using the control points in such a state.
The invention disclosed in D. Mattes et al., “Nonrigid Multimodality Image Registration”, Proceedings of the SPIE, Vol. 4322, pp 0.1609-1620, 2001 applies the non rigid registration process to images of the same subject obtained by a PET (Positron Emission Tomography) apparatus and a CT (Computed Tomography) apparatus. The invention disclosed in D. Mattes et al., employs mutual information as a measure of the degrees of similarity among pixel values within the two types of images.